Sorry for the wrong citation in previous message:
The crystallographic relation between rutile needles and garnet host is
given in the following paragraph in Hwang et al. (2007, JMG, 25, 349-362):
"The pronounced needle-like morphology of rutile also suggests that they
could have grown along a specific lattice direction of rutile, i.e. the long
axis of rutile needle could be parallel to a specific lattice direction of
rutile. For this reason, seven needles from samples 96Y5A/96Y5B were
analysed in detail in order to define their growth directions. Five needles
are within ~7o off from the normal of {101}rt (cf. Fig. 2f for example), one
needle is ~23o off from the normal of {101}rt and is nearly parallel to the
<101>rt, the other is ~61o off from the normal of {101}rt and is nearly
parallel to the normal of (210)rt. The preferred {101}-normal growth
explains the oblique extinction of the rutile needles in OM observations.
Note that the oblique extinction of oriented rutile inclusions in garnet was
previously taken as a probable indication of the presence of high-pressure
polymorphs of titania in other UHP rocks (Song et al., 2004). By plotting
the long axes of all needles along the [-11-1] direction of garnet,
stereographic projections of the rutile a- and c-axes show their random
orientation relative to the garnet host (Fig. 3). In other words, except for
the gross alignment between the <111>grt and {101}-normal of rutile, there
are no specific crystallographic orientation relationships between garnet
and rutile."
========================
Dr. Tzen-Fu Yui
Institute of Earth Sciences
Academia Sinica
P.O. Box 1-55, Nankang,
Taipei-, Taiwan
TEL: 886-2-27839910 ext 621
FAX: 886-2-27839871
e-mail: [log in to unmask]
=========================
-----Original Message-----
From: Metamorphic Studies Group [mailto:[log in to unmask]] On
Behalf Of Horst Marschall
Sent: Friday, December 09, 2011 11:38 PM
To: [log in to unmask]
Subject: Re: inclusions in garnet
On 9 Dec 2011, at 10:21, Jim Eckert wrote:
> Actually, a seemingly 'characteristic' feature of these seems to be
anomalous extinction...i.e., not parallel to c.
I assume that the extinction is not parallel to the elongation of the
needles? That doesn't necessarily mean that it is not parallel to the c axis
of the rutile. It could mean that the exsolution of the rutile is entirely
controlled by the host garnet, and the habitus of the lamellae doesn't
express their crystallographic orientation. Any insight from the TEM folks
on this?
cheers,
Horst
